Mixer circuits, or mixers, are widely used in modem communication systems to translate the frequency of an input signal up or down to an Intermediate Frequency (IF) where further signal processing and demodulation can occur. This translation function is referred to herein as a frequency down-conversion or up-conversion when applied to the signal path of a receiver or transmitter. In a receiver, a front end mixer multiplies the input Radio Frequency (RF) signal with an internal Local Oscillator (LO) signal to produce sum and difference frequencies (e.g., f(mixer out)=±f(RF)±f(LO)), one of which is chosen as the desired IF. The undesired response, referred to herein as an image frequency, is typically filtered out. The IF signal is more easily amplified, processed, and ultimately demodulated because the IF frequency is predetermined.
A common circuit for performing the mixer function in a receiver is a Gilbert Cell. In one example of a Gilbert Cell mixer circuit, an RF input signal is fed to a differential transconductance amplifier (e.g., Metal Oxide Semiconductor (MOS) differential amplifier) having a supply current determined by a bias source. The current in the bias source is typically established by a bias voltage (e.g., Vbias). The mixer core of the Gilbert Cell receives local oscillation signals at differential inputs (e.g., LO(+) and LO(−)). The differential transconductance amplifier modulates the differential current in the mixer core by the RF input signal (e.g., RF(+) and RF(−)). At a differential load coupled to the output of the mixer core, an output signal is provided consisting of the RF input signal switched by the mixer core at the rate of the LO frequency. The output signal corresponds to the frequency difference between the RF and LO signals.
During frequency conversion, the Gilbert Cell mixer may generate intermodulation (IM) distortion when multiple signals are present at the RF port. When the input and output circuits of the mixer are nonlinear, undesired multiplication of the RF and IF signals with each other occurs producing spurious frequencies which comprise the IM distortion component of the mixer output. Second-order intermodulation (IM2) and third-order intermodulation (IM3) distortions may pass to the mixer output and corrupt the instantaneous amplitude and phase of the desired IF signal. A third order IM intercept point (IIP3) for this Gilbert cell circuit is given by the following equation:
                              IIP          ⁢                                          ⁢                      3            dBm                          =                  10          +                      10            ·                          log              ⁡                              (                                                      32                    3                                    ·                                                            I                      ss                                                                                      μ                        n                                            ·                                              C                        ox                                            ·                                              W                        L                                                                                            )                                              -                      10            ·                          log              ⁡                              (                                                      R                                          i                      ⁢                                                                                          ⁢                      n                                                        50                                )                                                                        Eq        .                                  ⁢        1            where IIP3dBm is the third order IM intercept point expressed in decibels in relative to a milliwatt, Iss is a source current for the differential transconductance amplifier (e.g., MOS differential amplifier), μn is the carrier mobility of electrons in the MOS channel, Cox is a MOS gate capacitance per unit area, W is an effective width of the MOS channel, L is an effective length of the MOS channel, and Rin is a mixer RF port input resistance.
Eq. 1 illustrates an inverse relationship between IIP3 and the gain, which is proportional to W/L, as well as a positive proportionality of IIP3 to the supply current (Iss). In a low voltage, low power operating regime, such as may be found in a cellular communication device or a portable electronic device, it is desirable to maintain or improve gain while decreasing supply current demands. The design trade-offs for increasing mixer linearity at low voltage often sacrifices gain and low supply current drain.
Accordingly, a signal mixing circuit is desired that optimizes linearization under relatively low supply voltage and low supply power conditions. In addition, a method for signal mixing is desired that optimizes linearization under a relatively low supply voltage and low supply power condition. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.